E0135

A range pattern that cannot match

Error

Built from the repository at commit a8614dc.

A range pattern takes i64, u8 or char, and matches at least one value: 5..=4 and 5..5 match none.

Why

A range compares a value with its bounds, so it needs a type whose values are ordered: an integer, or a char by its code point. A string, bool or enum has no such order in a pattern. A range that matches no value is a mistake, usually bounds in the wrong order, and Rust rejects it too.

Broken

fn first_half(c: char) -> bool {
    match c {
        "a".."n" => true
        _ => false
    }
}

fn main() {
    print(first_half('c'))
}

What sisu says

error[E0135]: a range pattern takes `i64`, `u8` or `char`
 --> E0135.sisu:3:9
  |
3 |         "a".."n" => true
  |         ^^^^^^^^

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0135

Fixed

fn first_half(c: char) -> bool {
    match c {
        'a'..'n' => true
        _ => false
    }
}

fn main() {
    print(first_half('c'))
}